Galactic evolution: more data, no more answers

New results from digital sky surveys highlight more inconsistencies in our …

Over the past few years, the field of galactic evolution has been thrown a number of observational curve balls. Theory can describe galaxies that look like the ones in our general neighborhood of spacetime, but cannot describe the vastly different types of galaxies that appear to be the ancestors of modern ones.

The problem is that new data is suggesting that early galaxies were an extremely diverse bunch. Astronomical observations reveal that some early galaxies were massive, cranking out stars at a phenomenal rate, and growing ridiculously fast. Others appeared much later than they should, and now evidence suggests that still others are far smaller than predictions suggest. The latest observational data are published in the current edition of Nature, where researchers describe how early massive galaxies had enough stars to weigh in somewhere close to today's elliptical galaxies, but squeezed all of them into a much more compact form.

Recent evidence suggests that massive galaxies, the current day heavyweights, have grown about five-fold over the past 10 billion years. In order for their ancestral galaxies to have had these masses and relatively small physical sizes that we've observed, the constituent stars would need to be moving at a very high velocity in order to prevent a gravitational collapse.

The authors focused on galaxy 1255-0, which has light that is now reaching Earth after traveling about 11 billion light years—it's old enough to represent an ancestral galaxy. The galaxy's key properties are the total weight of its stars (the stellar mass), how fast they're orbiting its core (the velocity dispersion), and how much physical space it occupies. Using data obtained with the Gemini Near-Infrared Spectrograph, the authors derive a velocity dispersion of 510+165-95 km*s-1. Prior studies of this galaxy have derived a stellar mass of 2x1011 solar masses, while Hubble images estimate its diameter to be 0.78�0.17kpc.

That velocity distribution is very high when compared to other, more typical, galaxies in the nearby—and hence more recent—Universe. This high velocity dispersion corroborates prior evidence that hinted at the high mass of 1255-0. This confirmation of high stellar mass in a compact galactic space, however, leads to a big hole in our understanding of galactic evolution. How did these massive, dense, high redshift galaxies make the transition to the more disperse ones seen nearby us today?

The formation of a galaxy like 1255-0 is qualitatively consistent with models that suggest early galaxies formed through a highly dissipative process, although explaining how a galaxy such as 1255-0 formed without forming stars at more distant radii from its core is problematic.

As the authors point out, the simplest explanation for what appear to be massive, compact early galaxies is that the measurements are wrong. However, the data presented in this current letter makes that argument difficult to maintain, since different observational methods give consistent results. So, we're stuck with the alternative: early massive galaxies seemingly existed in a much denser form than they do today.

To attempt to explain the transition from old, compact galaxies to modern forms, the authors suggest that the most plausible mechanism is that of galactic mergers. In the paper's conclusion, though, the authors suggest that "it is an open question whether mergers alone can 'puff up' galaxies by the required amount, as the precise effect depends on the accretion rate, the masses, orbits and gas content of accreted galaxies, angular momentum transfer between stars and dark matter, and on possible evolution in the contribution of dark matter to the measured kinematics."

To answer these questions, the authors say that we may need to wait for the next-generation 8m telescopes and spectrographs, which should be able to "revolutionize" the field of study of galactic evolution in the next few years.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.